I. Field of the Invention:
This invention relates to a method for transmitting a given relatively high-resolution image from a transmitter to a receiver over a narrow-band communication channel for display of the image at the receiver.
II. Description of the Prior Art:
Ordinarily, a relatively high-resolution image (e.g., an image contained in each successive television frame), is transmitted to a receiver over a wide-band communication channel for display of the image at the receiver. However, there are several cases in which transmission of the high-resolution image to a receiver is restricted to a narrow-band communication channel. One such case is so-called "teletex," in which successive pixels (which are normally in digitally-encoded sample form, but could be in analog form) of a given high resolution image are transmitted only during one or two horizontal scan lines of the vertically-blanked portion of each of a plurality of successive television frames. Another case is so-called "videotex," in which such successive pixels (which are normally in digitally-encoded sample form, but could be in analog form) of a high-resolution image are transmitted (either in digital or analog form) over a voice-grade telephone line. A wide-band television channel transmits the entire relatively high-resolution video image contained in each NTSC television frame in only 33 milliseconds (ms). Because a narrow-band voice-grade telephone line only has about 1/2000 th the capacity of a wide-band television channel, the same image that takes only 33 ms to transmit over a television channel takes at least one minute over a voice-grade telephone line.
Usually the purpose of transmitting an image from a transmitter to a receiver is to display the image on the screen of a cathode-ray-tube (CRT), so that it may be observed and comprehended by a viewer. Watching an image appear slowly, on line at a time, on the CRT screen is very frustrating. This is particularly so if the viewer is trying to find a certain image in a sequence of images. Part of the reason for this is that when the image is comprised of pictorial information (which is often), it takes the transmission and display of about the first one-third of the image before the viewer is able to garner some idea of what type of picture information is contained in the image. This means that, in the case of a voice-grade telephone line, the viewer must view a mostly blank screen for at least about 20 to 30 seconds before he has sufficient usable information to decide whether or not the image then being displayed is one in which he may be interested.